There are known two types of methods for synthesis of lithium tetrafluoroborate (LiBF4), one type of which is known is called “wet method” and the other type of which is called “ether method”. In the wet method, lithium tetrafluoroborate is synthesized as a hydrate salt (LiBF4.H2O) by reaction of fluoroboric acid with lithium carbonate. Although the hydrate salt needs to be dehydrated by heating at about 200° C., the lithium tetrafluoroborate gets decomposed (LiBF4→LiF+BF3) during the heating and becomes low in purity. Further, some water remains in the lithium tetrafluoroborate. The lithium tetrafluoroborate obtained by the wet method cannot thus be used for lithium batteries. In the ether method, lithium tetrafluoroborate is synthesized as an anhydrous salt by reaction of a complex compound of boron trifluoride and methyl ether or ethyl ether with lithium fluoride. The ether method however has some drawbacks that: the lithium tetrafluoroborate is difficult to dissolve in ether and thereby difficult to secure a satisfactory level of quality for use in lithium batteries: the use of dangerous ether is essential etc.
On the other hand, Patent Document 1 discloses a method for producing lithium tetrafluoroborate, by reacting lithium fluoride with boron trifluoride in a solvent of chain carbonate ester and dissolving the resulting lithium tetrafluoroborate in the solvent.
Patent Document 2 discloses a method for purifying a lithium battery electrolytic solution containing lithium tetrafluoroborate, by converting various acidic impurities to hydrogen halide with the addition of a halide to the electrolytic solution, removing the hydrogen halide from the electrolytic solution and thereby providing the electrolytic solution with less impurities.